The present invention relates to a non-aqueous electrolyte battery.
In recent years, the market of portable data apparatus such as cellular phone, small-sized personal computer and portable audio equipment has been rapidly growing. These portable apparatus comprise a non-aqueous electrolyte secondary battery having a high energy density. The enhancement of the performance of the non-aqueous electrolyte secondary battery is still under study. These portable apparatus are produced on the supposition that they are carried by men. Therefore, these portable apparatus must exhibit their performance and assure safety in various atmospheres.
This type of a non-aqueous electrolyte secondary battery comprises a lithium-containing cobalt composite oxide or lithium-containing nickel composite oxide as a positive electrode material, a graphite-based or coke-based carbon material as a negative active material and a solution of a lithium salt such as LiPF6 and LiBF4 in an organic solvent as an electrolyte. The positive electrode and the negative electrode each are in the form of sheet. The two electrodes have the electrolyte retained therein. The positive and negative electrodes are disposed opposed to each other with an electrically insulating separator provided interposed therebetween. The laminate is received in a vessel having various shapes to form a battery.
In some unforeseen electrical uses such as overcharge, the aforementioned non-aqueous electrolyte secondary batteries undergo chemical reaction different from those occurring in usual charge-discharge process and become thermally unstable. In this case, it is likely that the electrolyte mainly containing a combustible organic solvent can be combusted to impair the safety of the batteries. Further, when the ambient temperature rises, the resulting vaporization of the inner electrolyte causes the rise of the inner pressure, making it likely that the rupture of the exterior material and concurrent ignition of the electrolyte can occur when the ambient temperature is too high. Moreover, when external impact, deformation or damage is given to the battery to cause the electrolyte to leak out, it is likely that the electrolyte can catch fire and combust because it is an inflammable liquid.
In order to solve these problems, the change of formulation of electrolyte has been studied. The related art organic solvent-based electrolytes have heretofore comprised ethylene carbonate, diethyl carbonate, ethyl methyl carbonate, γ-butyrolactone or the like as a solvent. The flash point of ethylene carbonate, diethyl carbonate, ethyl methyl carbonate and γ-butyrolactone are 152° C., 31° C., 24° C. and 98° C., respectively. In an attempt to enhance the safety of batteries, only ethylene carbonate or γ-butyrolactone, which has a relatively high flash point among these solvents, has been used. However, since it has been reported that the temperature of the interior of passenger cars is sometimes more than 100° C. in summer, these solvents are not sufficient. Further, when batteries comprising such a solvent are used at 60° C. or more, it is likely that the life of the batteries can be shortened or the battery vessel can be destroyed by the generation of gas in the batteries. In addition, the electrolyte is still combustible even if these solvents are used and thus can be combusted when it catches fire.
In an attempt to drastically enhance the safety of batteries, the use of room temperature molten salts having no flash point as electrolyte has been studied. However, molten salts have a high viscosity and hence a low ionic conductivity that gives extremely low output performance. These molten salts are also disadvantageous in that they can be difficultly impregnated into the positive and negative electrodes and the separator.
In order to solve these problems, the incorporation of a non-aqueous solvent which has been heretofore used, such as diethyl carbonate and ethylene carbonate in the molten salt has been studied. However, although the molten salt is incombustible or fire retardant, the incorporation of the combustible organic solvent is disadvantageous in that the safety, which is one of great advantages attained by the use of the molten salt, can be impaired.
Among various molten salts, molten salts containing tetrafluoroborate anion (abbreviated as “BF4−”) or bis(trifluoromethanesulfonyl)amide anion (abbreviated as “TFSI”) having a relative low viscosity leave something to be desired in cycle performances or retention of performances in a high temperature atmosphere such as 60° C. and exhibit drastically deteriorated output performances as compared with non-aqueous electrolyte batteries comprising organic solvents such as carbonate-based solvent which have been already put to practical use. Further, molten salts having a higher content of fluoroalkyl group such as bis(pentafluoroethanesulfonyl)amide anion (abbreviated as “BETI”) have a higher viscosity that causes a drastic deterioration of output performance or other performances. Moreover, JP-A-2002-110225 proposes that a lithium salt should be incorporated in a molten salt in an amount as small as from 0.2 to 1.0 mol/L to keep the ionic conductivity as high as possible. However, the incorporation of the lithium salt causes the viscosity of the molten salt to rise more than that of the molten salt itself and the ionic conductivity of the electrolyte to fall, making the drastic deterioration of output performances and cycle performances unavoidable.
The related art non-aqueous electrolyte batteries cannot be expected to exhibit enhanced output performances and cycle performances because the electrolyte containing the room temperature molten salt used has a high viscosity and thus can permeate the separator too difficultly to make effective use thereof.